Multiplication stimulating activity (MSA), a 10,000 molecular weight polypeptide synthesized by a cultured rat liver cell line; exhibits remarkable similarities with somatomedins purified from human plasma -- somatomedin A, NSILA I and II -- in physical and biological properties. Extensive comparison of these peptides in multiple specific radioreceptor assays confirms this high degree of relatedness MSA also has overlapping reactivity with insulin. In addition, both peptides are capable of reacting with some MSA receptors and all insulin receptors. The MSA and insulin receptors of human fibroblasts appear particularly closely related, since antibodies to insulin receptors also react with MSA receptors. BIBLIOGRAPHIC REFERENCES: Rechler, M.M., Bruni, C.B., Podskalny, J.M., Warner, W., and Carchman, R.A.: Modulation of serum-stimulated DNA synthesis in cultured human fibroblasts by cAMP. Experimental Cell Research 104: 411-422, 1977. Rechler, M.M., Podskalny, J.M., and Nissley, S.P.: Characterization of the binding of multiplication stimulating activity (MSA) to a receptor for growth polypeptides in chick embryo fibroblasts. J. Biol. Chem. 252: 3898-3910, 1977.